In a previously known device for solving the problem of providing the wire with adjustable length and approximately constant tension, one end of the wire was made to co-operate with a hydraulic stretching device. This device has not proved to be completely successful, because the wire tension produced by the pressure fluid in the system could not always be ensured to be sufficient. There is a risk, that the hydraulic system at certain grapple movements and by internal leakage at times is not capable of supplying the wire stretcher with pressure fluid to maintain the wire sufficiently stretched.